<!DOCTYPE html>
<html>
<head>
  <title>可编辑的输入框 </title>
  <style>
    #editableDiv {
      border: 1px solid #ccc;
      padding: 10px;
      min-height: 100px;
    }
    #prefix {
      color: gray;
    }
  </style>
</head>
<body>
  <div id="editableDiv" contenteditable="true" oninput="handleInput(event)">
    <span id="prefix">问你想问...</span>
  </div>
  

  <script>
    function handleInput(event) {
      var prefix = document.getElementById('prefix');
      var editableDiv = document.getElementById('editableDiv');
      if (editableDiv.innerText === '') {
        prefix.style.display = 'inline';
      } else {
        prefix.style.display = 'none';
      }
    }
  </script>
</body>
</html>
